A fundamental step in calculating PV performance is determining the irradiance incident on the plane of the array (POA) as a function of time. This POA irradiance is dependent upon several factors, including: Sun Position; Array Orientation (fixed or tracking) Irradiance Components (Direct and Diffuse) Ground Surface Reflectivity

There exist many models to estimate POA irradiance from the standard components (GHI, DNI, and DHI). We describe a few of the more popular models in the next several pages.

The model for ground reflected irradiance $$E_g$$ develops from the following assumptions: The array is infinitely long. Irradiance on the ground is uniform and equal to $$GHI$$, i.e., horizon blocking and near-field shading by the array is ignored.

Effective irradiance is total plane of array (POA) irradiance adjusted for angle of incidence losses, soiling, and spectral mismatch. In a general sense it can be thought of as the irradiance that is “available” to the PV array for power conversion.

$$DHI$$ is diffuse horizontal irradiance, $$AOI$$ is the angle of incidence between the sun and the plane of the array. $$\theta_Z$$ is the solar zenith angle. $$\theta_T$$ is the array tilt angle from horizontal. $$F_1= \max \left[ 0,\left( f_{11}+f_{12} \Delta + \frac {\pi \theta_Z}{180^{\circ}}f_{13} \right) \right] $$,

Diffuse radiation from the sky dome is typically divided into several components: the isotropic component, which represents the uniform irradiance from the sky dome; the circumsolar diffuse component, which represents the forward scattering of radiation concentrated in the area immediately surrounding the sun; the horizon brightening component.

Irradiance & Insolation. Extraterrestrial Radiation; Air Mass; Direct Normal Irradiance. Piecewise Decomposition Models; DIRINT Model; Global Horizontal Irradiance; Diffuse Horizontal Irradiance; Spectral Content. AM 1.5 Standard Spectrum; Weather Data Sources for Performance Modeling. Global Normal Spectral Irradiance Dataset from Albuquerque ...

The isotropic sky diffuse model is the simplest of the POA sky diffuse models and forms the foundation upon which more accurate models build. The isotropic sky diffuse model assumes that the diffuse radiation from the sky dome is uniform across the sky.

The Hay and Davies diffuse model divides the sky diffuse irradiance into isotropic and circumsolar components. Horizon brightening is not included. An anisotropy index, $$A_i$$ , is defined as: $$A_i=\frac{DNI}{E_{a}}$$, where $$DNI$$ is the direct normal irradiance and $$E_a$$ is the extraterrestrial radiation.

Irradiance is an instantaneous measurement of solar power over some area. The units of irradiance are watts per square meter. For practical purposes of measurement and interpretation, irradiance is expressed and separated into different components.